In conventional piston type refrigerant compressors, pistons reciprocate in cylinders which are generally made of iron casting. Iron casting is utilized because iron is durable and resists abrasion. These characteristics of iron have been especially important in the case of a compressor for an automotive air conditioning system. Generally, iron cylinders, or alternatively a cylinder liner made of iron casting, are inserted into a casing made of an aluminum alloy.
When a cylinder or cylinder liner made of iron casting is used in a compressor, the weight of the cylinder or cylinder liner cannot be decreased more than a certain amount. The cylinder or cylinder liner must be more than a certain thickness to achieve sufficient strength to avoid any deformation when the cylinder liner is inserted in the casing. The cylinder or cylinder liner also must be thick enough to prevent the cylinder liner from undergoing heat deformation. However, when the cylinder liner is made sufficiently thick to achieve the above advantages, the weight of the casing including the cylinder liner is larger than desired. Also, since the above type cylinder liner involves two manufacturing steps, i.e., manufacturing a separate iron cast cylinder liner followed by the assembly of the cylinder liner and casing, the manufacturing cost is high.
To resolve the above mentioned problems, cylinder liners have been made of the same aluminum alloy used in the casing. However, the use of an aluminum alloy in the cylinder liner makes it difficult to use piston rings in the compressor. Piston rings generally are disposed on the outer surface of the pistons to improve the seal between the cylinder chamber and crank chamber of the compressor. Piston rings generally cannot be used in cylinder liners made of an aluminum alloy, because piston rings generally have high hardness as compared to the hardness of aluminum cast cylinder liners. Hard piston rings damage the softer aluminum cylinder liner.
Referring to FIG. 6, a portion of a wobble plate type compressor is shown. Piston 16', which is connected to connecting rod 15', piston ring 17' and cylinder liner 19' are shown. In a wobble plate type compressor, piston 16' does not reciprocate perfectly, as depicted by the slightly slanted position of the piston in FIG. 6, because connecting rod 15' connected to piston 16' does not remain parallel with the center line of cylinder liner 19' throughout its cycle. Side pressure F is produced on outer surface 18' of piston 16' to force the bottom end of piston 16' toward cylinder liner 19'. Accordingly, the side surface of piston 16' can contact the cylinder liner and damage the liner during reciprocation of the piston.
To improve upon the above problems, an improved piston construction was previously proposed in Japanese Utility Model Application No. 58-197942 corresponding to U.S. application Ser. No. 684,332, now U.S. Pat. No. 4,594,055, issued June 10, 1986, which is assigned to the same assignee as the present application. In the '942 application, two grooves are formed on the outer surface of a piston at the top and bottom thereof. Piston rings, which are made of plastic and conical in shape, are disposed in those grooves. Although the inclination of the piston is resolved by the above construction, the edges of the piston rings still can slide on the surfaces of the cylinder liners causing abrasion between the piston rings and the cylinder liner.